This invention relates to a shut down method for a fuel cell system and a fuel cell system itself.
As is well known, a fuel cell operates to react an enriched fuel such as hydrogen with air so as to generate electrical energy through the stack of a fuel cell. The hydrogen fuel is obtained by reforming a methanol/water mixture in a reformer that is comprised of a catalyst and a heater for vaporizing the unreformed fuel. Fuel cells are generally employed as stationary power units wherein they are operated for long periods of time. There are, however, applications wherein it would be desirable to employ a fuel cell as a source of electrical power during intermittent operation, such as in the operation of a small electrically powered vehicle. As such, the fuel cell must be not only compact but must be capable of being turned on and off more frequently than stationary power supplies. Therefore, the shutdown for the fuel cell is particularly important and the shutdown of the reformer can be a significant part of this operation.
Specifically, when the reformer is shut off, the temperature of the reformer and specifically the catalyst bed reduces only gradually. Because of this slow cool down of the reformer and specifically its catalyst bed, methanol and water vapor in the reformer can be condensed so as to create a negative pressure within the reformer. Although valves are normally employed for closing off the reformer from the atmosphere when the system is shut down, air can easily leak back into the reformer under such negative pressure conditions and cause oxidation and deterioration of the catalyst material.
In order to prevent such deterioration of the catalyst bed, systems have been proposed wherein the reformer is purged with an inactive gas such as nitrogen until the catalyst temperature becomes sufficiently low that condensation and deterioration are not a problem. However, such systems require a source of the inactive or inert gas and the necessary valving and pressure regulations for them. Therefore, this type of system becomes quite complicated and bulky.
Another method of protecting the catalyst bed has been to fill it with fuel once the system is shut down. However, the surrounding of the catalyst with fuel when the system is shut down can give cause to problems, particularly on subsequent start ups. The subsequent start up of the catalyst that has been filled with fuel can cause significant expansion on restart which can interfere with the catalyst bed either by deteriorating it or by altering its state of packing, thus making it less efficient.
It is, therefore, a principal object of this invention to provide an improved fuel cell system and reformer arrangement for such a system.
It is a further object of this invention to provide an improved method for shutting down the reformer of a fuel cell system.
It is a further object of this invention to provide an arrangement for more rapidly lowering the temperature of the catalyst bed of a reformer for a fuel cell during shutdown so as to avoid deterioration or damage to the catalyst material and bed.